1. Field of the Invention
The present invention relates to a liquid crystal material, and more particularly to a ferroelectric charge-transport liquid crystal material, that exhibits ferroelectric liquid crystallinity and, in addition, charge-transport properties, and various elements or devices using the material.
2. Background Art
Liquid crystal materials having various structures are known in the art, and have been widely used mainly as materials for information display devices using electro-optic effect based on the alignment effect of liquid crystal molecules attained by application of voltage. Further, application of liquid crystal materials to optical shutters, optical stops, modulating devices, lenses, light beam deflection/optical switches, phase diffraction gratings, optical logic devices, memory devices and the like are under study.
External stimulation by heat, electric field, magnetic field, pressure or the like results in transition of the alignment of liquid crystal molecules which enables optical properties, electric capacity and the like of the liquid crystal to be easily changed. Sensors and measuring instruments, utilizing these properties, for temperature, electric field/voltage, infrared radiation, ultrasonic wave, flow rate/acceleration, gas or pressure have been studied in the art.
Charge-transport materials, wherein charge-transport molecules which serves as a charge-transport site are dissolved or dispersed in a matrix material, such as a polycarbonate resin, or charge-transport materials, wherein a charge-transport molecule structure pendent on a polymer main chain, such as polyvinyl carbazole, are known in the art. These materials have been extensively used as materials for photoreceptors in copying machines, printers and the like.
When the conventional charge-transport materials are dispersive charge-transport materials, what is desired for improving the charge-transport capability is high solubility of charge-transport molecules in polymers as the matrix. In fact, however, a high concentration of the charge-transport molecule in the matrix causes crystallization of the charge-transport molecule, and, hence, the upper limit of the concentration of the charge-transport molecule in the matrix is generally 20 to 50% by weight, although it depends upon the type of the charge-transport molecule. This concentration means that not less than 50% by weight of the whole material is accounted for by the matrix not having the charge-transport properties. This concentration thus poses a problem because, in the form of films, the charge-transport properties and the response speed are restricted by the matrix and hence are unsatisfactory.
On the other hand, in the case of the pendant type charge-transport polymer, the proportion of the pendant having charge-transport properties is high. This polymer, however, involves many practical problems associated with mechanical strength, environmental friendliness and durability of the formed film, and film-forming properties. In this type of charge-transport material, the charge-transport pendants are locally present close to one another, and this portion, when charges are hopped, serves as a stable site and functions as a kind of trap, unfavorably resulting in lowered charge mobility.
All the above amorphous type charge-transport materials raise a problem that, unlike crystal materials, the hopping site fluctuates in terms of space, as well as in terms of energy. For this reason, the charge transport properties depend greatly upon the concentration of the charge-transport site, and the carrier mobility is generally about 1.times.10.sup.-6 to 1.times.10.sup.-5 cm.sup.2 /Vs which is much smaller than that of molecular crystals, 0.1 to 1 cm.sup.2 /vs. Further, the amorphous materials have an additional problem that the charge-transport properties depend greatly upon temperature and field strength.
This is greatly different from charge-transport crystal materials. Charge-transport polycrystalline materials are promising materials in applications where a charge-transport layer having a large area is necessary, because it can form an even charge-transport film having a large area. The polycrystalline materials, however, are inherently uneven from the microscopic viewpoint, and present problems including that defects formed in the interface of particles should be controlled.
Accordingly, it is an object of the invention to provide a novel charge-transfer material which can solve the problems of the prior art, that is, possesses both advantages of amorphous materials, structural flexibility and evenness over a large area, and advantages of the crystalline materials, molecular alignment, and at the same time can realize the control of charge-transport properties by an external field and possesses high level of charge-transport properties, thin-film forming properties, various fastness properties and the like.
According to one aspect of the present invention, there is provided a ferroelectric charge-transport liquid crystal material comprising a liquid crystal compound, said liquid crystal material having a carrier mobility of not less than 10.sup.-5 cm.sup.2 Vs, and applications thereof.
The ferroelectric charge-transport liquid crystal material of the present invention has a self-aligning property by virtue of the molecular structure. Therefore, unlike the molecule dispersed material, use thereof as a hopping site inhibits spatial and energetic dispersion of the hopping site and can realize band-like transport properties, that is, electron conduction, such as found in molecular crystals. As such, a larger mobility than that in the conventional molecule dispersed materials can be realized and the mobility does not depend upon the electric field. Further, by virtue of the self-polarization, the self-aligning properties can be controlled by an external field, and a change in property values as a result of the control of the self-aligning properties can also be controlled. That is, the ferroelectric charge-transport liquid crystal material has liquid crystallinity and at the same time can transport charges in response to visible light. Therefore, the ferroelectric charge-transport liquid crystal material is useful in the applications of the conventional liquid crystal, as well as in materials for applications utilizing charge-transport properties, such as photosensors, electroluminescence devices, photoconductors, space modulating devices, thin-film transistors, photorefractive devices, and other sensors. In particular, the ferroelectric charge-transport liquid crystal material according to the present invention has excellent sensitivity to visible light, and hence is useful as materials for photosensors.